Well... datasets are not pools... you'll be be better off creating a single RAIDZ2 pool with all 6 of your 3TB disks connected to the 6 SATA ports on your motherboard. Plan on booting FreeNAS from a USB stick; if you're paranoid, use 2 USB sticks and mirror the FreeNAS installation (the installation program allows you to do this).
Then you can create datasets on this pool for media, apple time machine backups, and whatever else you need. You can configure quotas on the apple time machine dataset to limit how much space it uses. Realistically, this is the only quota you need to worry about.
For a home user, there really isn't any good reason to create multiple pools when you only have 6 disks to work with.
My though was that i can save some space for important data. Let's see if that is the case:
5 x 3 TB gives[1] 9.446 TB usable data, that i would spend only for important stuff.
6 x 3 TB gives 11.81 TB usable data. Minus, say, 500Gb for each TimeMachine and same for iTunes videos leaves 10.31 TB for important stuff. So it looks like you are right indeed and I am better of with a single pool of 6 drives.
Thanks for the advice!
[1] https://forums.freenas.org/index.php?threads/zfs-raid-size-and-reliability-calculator.28191/
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